1. Field of the Invention
The present invention relates to an optical interconnection to be utilized in interconnecting a plurality of micro-processors in a highly parallel computer system.
2. Description of the Background Art
A parallel processing by a plurality of micro-processors has been known to be effective in improving the processing speed of a computer system. Recently, the research in this direction has advanced to the level of developing a highly parallel computer system using thousands of micro-processors in parallel.
In realizing such a highly parallel computer system, the interconnection among the parallel processing micro-processors becomes a critical factor. Namely, in a highly parallel computer system, the implementation of all the micro-processors involved on a single processor board is not only highly improbable physically speaking, but also unfavorable from a point of view of interconnection, so that the micro-processors are going to be divided into a plurality of groups to-be implemented on a plurality of processor boards separately. Accordingly, it is important for the interconnection among the micro-processors in the highly parallel computer system to be able to provide smooth data transmissions between the micro-processors on different processor boards.
Moreover, the recent micro-processor has a quite high processing speed itself and the data to be transmitted to and from such a recent micro/processor also require the high speed and large capacity transmission, so that the interconnection among the micro-processors of a high performance highly parallel computer system is also required to be capable of the high speed and large capacity data transmissions.
For these reasons, the interconnection shown in FIG. 1 has been employed in a conventional highly parallel computer system. In this interconnection configuration of FIG. 1, the micro-processors 82 and 83 on the adjacent processor boards 80 and 81 are interconnected by means of electrical wirings 84 physically arranged between these micro-processors 82 and 83. However, in such a conventional interconnection configuration, there are problems such as a number of wirings necessary for interconnecting the adjacent processor boards becomes enormous, the high speed data transmission becomes difficult, and the data transmission between arbitrary two micro-processors may require the data to be relayed through a large number of intermediating micro-processors.
In order to cope with such problems, recently, there is a proposition to replace the conventional electrical wiring interconnection by an optical interconnection, which has the advantage of being able to realize a simplified interconnection configuration and a high speed data transmission.
In realizing such an optical interconnection in a highly parallel computer system using more than two processor boards, for a case of transmitting the data between two not adjacent processor boards, it is necessary for the data to be relayed through at least one intermediate processor board. For this reason, the optical interconnection requires the use of a light transmission element or an optical element combining a light intercepting element and a light emitting element. The optical interconnection as a whole is then constructed by providing an optical device having a plurality of such optical elements arranged in a planar array for each processor board of the highly parallel computer system. The similar optical interconnection is also applicable to a system for dealing with a large amount of data transmissions such as a large capacity exchanger.
Now, the quality of the integrated optical element planar array of such an optical interconnection such as its performance, realizability, and easy handling largely depends on the optical elements used. Conventionally, as a light transmission element to be used in the optical interconnection, a planar semiconductor laser amplifier has been used, and as an optical element combining a light intercepting element and a light emitting element, an optical element shown in FIG. 2 in which a light emitting element 85 and a light intercepting element 86 are arranged side by side has been used.
The optical element shown in FIG. 2 is normally capable of intercepting the light signals from the upper side of the element and emits the light signals toward the upper side of the elements, but is also capable of intercepting the light signals from the lower side of the element under appropriate conditions. However, when the optical element planar array is formed from a plurality of such optical elements, in order to align the light emitting portion of the optical element on one processor board with the light intercepting portion of another optical element on adjacent processor board, either the processor boards 87 must be arranged with mutual displacement as shown in FIG. 3, or the number of optical elements on each processor board must be limited and the emitted light beam 88 must have a wider radiation angle as shown in FIG. 4.
On the other hand, in a case of using light transmission elements, the transmitted light signals have not been directed toward the light intercepting portion of the optical elements on the other processor boards, so that the realization of the interconnection among a plurality of processor boards has been difficult.
Furthermore, the emitted light signals tend to have the widening beam diameter due to the diffraction, so that the light signals emitted from one processor board can be irradiated onto the portion other than the light intercepting portion of the adjacent processor board, and such light signals irradiated onto the portion other than the light intercepting portion can be reflected back to the original processor board to disturb the operation characteristics of the light-transmission elements or the light emitting elements located thereon, or cause the excitation of the photo-electrons in the adjacent processor board which in turn gives rise to the noises to affect the operation of the light intercepting elements or the light emitting elements located thereon.
Now, conventionally, a system having a plurality of processor boards has been formed by using a backplane (mother board) or cables. However, in a case of a highly parallel computer system in which a plurality of micro-processors must be provided on each processor board and a plurality of transmission paths must be provided between the micro-processors on the different processor boards, it has been necessary to reduce the number of wirings by using a sparse topology or a reduced bit width because of the limitation on the number of wirings that can be taken out from one side of the processor board to the backplane or cables, so that it has been difficult to secure the satisfactory transmission performance.
Moreover, for the electrical wirings for interconnecting the separated processor boards in a large system such as a highly parallel computer system, a much tighter timing requirement must be imposed compared with the connection within the board, and such electrical wirings are easily affected by the noises, so that it has been difficult to realize a high speed transmission frequency. In order to cope with such problems, it is necessary to increase the number of electrical wirings, but as already mentioned above there is a limit to a number of electrical wirings that can be accommodated, so that the improvement by increasing the number of electrical wirings has been difficult.
Furthermore, in a case of using cables, a large number of line drivers would be required, so that it is unfavorable from a point of view of large scale integration as well as from a point of view of energy consumption.
In these regards, the use of the optical interconnection has the significant advantages that the bandwidth is wider, it is high speed as the delay due to the floating capacity is small, there is no need for grounding, and it is not easily affected by noises.
For this reason, the optical LAN (Local Area Network) has been developed. Also, there has been a case of realizing a digital exchanger system in which the interboard connection is realized by using optical fibers. However, these systems uses rather small number of light signals, so that it is difficult to significantly improve the transmission speed of the medium speed electrical signals in the backplane. In addition, even when it is possible to replace a large number of electrical signals by a small number of light signals, this in turn gives rise to a problem of an additional hardware required for the multiplexing and distributing of the light signals.
There has also been a proposition in which the interconnection among the micro-processors in the highly parallel computer system is realized by the light beams propagating through a free space. However, such an optical interconnection has been associated with problems that it is necessary to change the holograms, it is necessary to move the light intercepting elements manually, and it is necessary to employ the time consuming liquid crystal switch for the switching of the connection.
There has also been a proposition for using a backplane with the light wave guides provided. However, the use of the backplane is associated with a problem that the maximum system size is limited by the size of the backplane.
There has also been a proposition to realize the optical interconnection among the micro-processors by using the light beams propagating through the free space between mutually facing light emitting element array chips and light intercepting element array chips. However, it has been difficult in this type of optical interconnection to realize the connection between not adjacent processor boards.
There has also been a proposition to realize the optical interconnection by using the light beams propagating through the free space within a cylindrical mirror configuration. However, this type of optical interconnection has the drawback that the number of processor boards that can be interconnected would be limited unless the mirrors are located quite far away because the reflection angle is limited, but the alignment of the mirrors becomes difficult as the mirrors are located far away. Furthermore, the connection configuration realized by this optical interconnection is that in which the logical sums of the output light signals of the processors are taken, so that it is far inferior to the cross bar connection.